Conventionally, an engine provided therein with an EGR device (exhaust gas recirculation system) which flows back a part of exhaust gas to intake air is known. By flowing back the exhaust gas whose oxygen concentration is low (EGR gas) to the intake air by the EGR device, combustion temperature is reduced so as to suppress generating of nitrogen oxide. A weight of the EGR gas flowed back to the intake air (target EGR gas flow rate) is calculated from a target flow rate map based on a differential pressure of an intake pressure and an exhaust pressure (intake/exhaust differential pressure) and an opening degree of an EGR valve controlling a flow rate of the EGR gas (EGR valve opening degree). For example, an art described in the Patent Literature 1 is so.
In the EGR device described in the Patent Literature 1, the EGR gas weight is calculated from one target flow rate map. Namely, the EGR gas weight is calculated uniquely from the intake/exhaust differential pressure and the EGR valve opening degree. However, influence caused by variation of a pressure ratio of the intake pressure concerning the exhaust pressure (intake/exhaust pressure ratio) applied to the calculated EGR gas weight is increased following approach of the pressure ratio to 1. Driving state of the engine is not determined uniquely from the intake/exhaust differential pressure and the EGR valve opening degree. Namely, though the driving state of the engine is different, the same EGR gas weight is calculated when the intake/exhaust differential pressure and the EGR valve opening degree are fixed. Accordingly, in the engine, a calculation error of the EGR gas weight may be increased by influence of the intake/exhaust pressure ratio and influence of an error of the driving state of the engine assumed from the intake/exhaust differential pressure and the EGR valve opening degree.